One of the key steps during the pathogenesis of atherosclerosis and abdominal aortic aneurysm (AAA) is the vessel wall remodeling, which involves extensive extracellular elastin and collagen degradation. Our laboratory has demonstrated that elastolytic cathepsins (Cat) S, K, and L are highly expressed during the progression of these diseases. Deficiency of Cat S results in significantly attenuated atherogenesis. However, it remains uncertain if other elastolytic cathepsins (e.g. K and L) are playing a similar role in atherogenesis and/or AAA, although their expression profiles in these lesions suggest this potential. To examine these possibilities, we proposed two specific aims to test our central hypothesis that the cysteine proteases Cat S, K, and L play critical roles in atherosclerosis and AAA and the regulation of their expression directly affects the pathogenesis of these diseases. We will first examine how and by which inflammatory cytokine(s) Cat K and L are regulated in primary cultured human and mouse vascular cells followed by directly examining their roles in mouse atherosclerosis and AAA models. Our preliminary data demonstrated that cysteine protease inhibitor cystatin C is deficient in lesions from human atheroma and AAA. By contrast, the pro-inflammatory cytokine macrophage migration inhibitory factor (MIF) is highly expressed in these lesions as well as in sera from patients with AAA. A role for MIF in cultured endothelial cells and macrophages is implicated in regulating cysteine protease expression. We will thus examine if deficiency of MIF or cystatin C will affect atherosclerosis and AAA in mice and study how MIF regulates cysteine protease expressison. The availability of animal models of atherosclerosis and AAA and different gene deficient mice of cysteine proteases and their regulators in the P.l.'s laboratory and the promising observations from the Cat S null mice completed by this group make this proposal practical, crucial, and unique. Data from these specific aims will not only delineate a molecular/cellular mechanism for these elastolytic cathepsins involving in atherosclerosis and AAA, but also lead to the discoveries of novel drug targets against these life threatening diseases as potent and selective small molecule cathepsin inhibitors are readily available.